With the ever-increasing popularity and competitiveness of golf, substantial effort and resources are currently being expended to improve golf clubs so that increasingly more golfers can have more fun and more success at playing golf. Much of this improvement activity has been in the realms of sophisticated materials and club-head engineering. For example, modern “wood-type” golf clubs (notably, “drivers” and “utility clubs”), with their sophisticated shafts and metal club-heads, bear little resemblance to the “wood” drivers, low-loft long-irons, and higher numbered fairway woods used years ago. These modern wood-type clubs are generally called “metal-woods.”
An exemplary metal-wood golf club such as a fairway wood or driver typically includes a shaft having a lower end to which a hollow club-head is attached. The club-head usually is made, at least in part, of a light-weight but strong metal such as titanium alloy. The club-head comprises a body to which a strike plate (also called “face plate”) is attached or integrally formed. The body includes a hosel that extends generally upward and is connected to the shaft of the club. The body also includes a heel region situated close to the hosel, a toe region situated opposite the heel region, a sole (lower) region, and a crown (upper) region. The body bears most of the impact load imparted to the strike plate when the club-head strikes a golf ball. The strike plate defines a front surface or strike face that actually contacts the golf ball.
In contrast to wood-type clubs used years ago, the club-heads of many modern metal-woods are hollow, which has been made possible by the use of light-weight, strong metals and other materials for fabricating the club-head. Use of titanium and other light-weight metal alloys has permitted the walls of the club-head to be made very thin, which has permitted the club-heads to be made substantially larger than their predecessors. These oversized club-heads tend to provide a larger “sweet spot” on the strike plate and higher club-head inertia, thereby making the club-heads more “forgiving” than smaller club-heads. This “forgiveness” means that a golfer using the club who strikes the ball off the center, or “sweet spot,” of the club's strike plate still produces a ball trajectory that is substantially similar to the shot that otherwise would have been made if the golfer struck the ball on the sweet spot. Characteristics such as size of the sweet spot are determined by many variables including the shape profile, size, and thickness of the strike plate as well as the location of the center of gravity (CG) of the club-head.
There are practical limits to the maximum size of club-heads, based on factors such as the particular material of the club-head, the mass of the club-head, and the strength of the club-head. Since the maximal mass of the club-head is limited under USGA rules, as the club-head size is increased, the walls of the body and face plate generally are made correspondingly thinner. The distribution of mass around the club-head typically is quantified by parameters such as rotational moment of inertia (MOI) and CG location. Club-heads typically have multiple rotational MOIs, each associated with a respective Cartesian reference axis of the club-head. A rotational MOI is a measure of the club-head's resistance to angular acceleration (twisting or rotation) about the respective reference axis. The rotational MOIs are related to, inter alia, the distribution of mass in the club-head with respect to the respective reference axes. Specifically, a wood-type club-head has a first rotational MOI about an x-axis (a horizontal heel-toe axis extending through the CG generally parallel to the face), a second rotational MOI about a z-axis (a vertical axis also extending through the CG), and a third rotational MOI about a y-axis (a horizontal front-back axis orthogonal to the x- and z-axes and also extending through the CG). The third rotational MOI usually is less significant than the other two. Here, “horizontal” is relative to the ground whenever the club-head is at address position relative to the golf ball. Each of these rotational MOIs desirably is high to provide the club-head with more forgiveness.
To achieve high rotational MOIs, and thus more forgiveness, the mass of the club-head typically is distributed, as much as possible, around the periphery of the club-head and rearward of the face plate. As a result, the club-head's CG generally is located rearwardly from the face plate at a prescribed location, which also helps the club to produce a desired launch angle upon impact with a golf ball.
Another factor in club-head design is the face plate. Impact of the face plate with the golf ball causes deflection of the face plate. This deflection and the subsequent recoil are measured as the club-head's coefficient of restitution (COR). A thinner face plate generally deflects more at impact than a thicker face plate of the same material. Thus, a club-head having a thin face plate can impart more energy and thus a higher initial velocity (rebound velocity) to a struck golf ball than a club with a thicker, more rigid face plate. This rebound phenomenon is called the “trampoline effect” and is an important determinant of the flight distance of the struck ball. Since face-plate deflection is usually greater in the sweet spot of the face plate, a ball struck by the sweet spot generally will have a higher rebound velocity than a ball struck off-center. Because of the importance of the trampoline effect, the COR of clubs is limited under USGA rules.
To achieve these ends, it typically is desirable to incorporate thin walls, including the face plate, into the designed configuration of the club-head. Thin walls also allow additional leeway in distributing club-head mass to achieve a desired mass distribution and a desired high COR.
The mass and volume of metal wood-type drivers are governed by USGA rules. Certain types of metal wood-type club-heads are quite large and have a volume that is equal to or nearly equal to 460 cm3, which is the maximum allowed by the USGA. These clubs typically have a large strike face that presents a tall face height to the ball. Consequently, with many golfers using these clubs, there is an increased probability that the ball will be struck by the strike plate at a location other than the sweet spot. These off-center shots deliver substantial stresses to the club-head to angularly pivot about the x-axis and/or z-axis. To make the club-head more resistant to such pivoting in response to these stresses, these large club-heads (indeed all club-heads) must have sufficient respective rotational MOIs about the CG of the club-head.
Regarding the total mass as the mass budget for the club-head, it is axiomatic that at least some of the mass be dedicated to achieving the required strength and structural support of the club-head. This is termed “structural” mass. Any mass remaining in the budget is called “discretionary” or “performance” mass, which can be distributed within the club-head to maximize performance. Much of the current research and development activity concerning golf clubs is directed to various ways of distributing the discretionary mass. For example, some club-heads include one or more weights placed relative to the heel-toe (x) axis and in-line with the percussion axis of the club-head. This manner of “perimeter weighting” can increase the rotational MOI of the club-head about the vertical (z) axis and increase the rotational MOI about the x-axis.
As club-head engineering converges on certain basic arrangements of discretionary mass in a club-head, particularly in metal-woods, achieving a maximal amount of any remaining discretionary mass is becoming increasingly important. It is also becoming more difficult to find sources of discretionary mass in the club-head that can be positioned advantageously. One general approach has focused on removing some mass from the strike plate while maintaining a uniform strike-plate thickness but without compromising the performance (e.g., stiffness) or durability of the strike plate. Unfortunately, if too much mass is removed from the strike plate under these conditions, the structural mass of the strike plate may be excessively compromised, which can result in the strike plate becoming too fragile and/or its COR becoming too high. Problems may also arise from stresses evenly distributed across the club-head upon impact with a golf ball, particularly at junctions of the face plate with other club-head components. In other words, many of these schemes are unsatisfactory, at least with certain club-heads.
In view of the above, various approaches have been investigated involving alteration of the face-plate configuration. For example, reference is made to U.S. Pat. Nos. 6,800,038; 6,824,475; 6,904,663; and 6,997,820, all incorporated herein by reference. Essentially, these references discuss various schemes for altering the thickness profile of the sweet spot. Specifically, on the rear (inner) surface of the strike plate, the “center” of the strike plate is relatively thin and is surrounded by an annular ridge, thereby giving the sweet spot a “volcano”-like thickness profile. The remainder of the strike plate, peripheral to the annular ridge, is of substantially uniform thickness or is made progressively thinner with increased radius from the center. Whereas these configurations showed some promise for some applications, they were not satisfactory in other applications.